This invention relates generally to fiber optic waveguides and particularly to fiber optic waveguides whose cores have two refractive indices and, still more particularly, to form birefringent optical fibers for propagating a single mode of optical energy in a defined polarization.
Wave behavior in layers of materials having different refractive indices optically and periodically stratified differs from that in a uniform medium. If the thickness of each layer is sufficiently small compared to the light wavelength and the number of layers is sufficiently large, the compound medium is birefringent. Form birefringence occurs in an ordered arrangement of layers of optically isotropic materials having dimensions large compared with the molecules of the materials, but small compared to the optical wavelength in the fiber. Fiber optic devices using form birefringent fiber are useful in constructing gyroscopes, sensors, frequency shifters and communications systems.
Problems arise in using ordinary fibers to form the above listed devices. Strictly speaking, an ordinary axially symmetrical single mode fiber is a "two-mode" fiber because it will propagate two orthogonally polarized HE.sub.11 modes. Each polarization has a propagation constant, but in an ordinary optical fiber the two propagation constants are so nearly identical that degeneracy results. Geometrical perturbations in the fiber cause polarization state instability when two orthogonal polarizations exist. Propagation of degenerate polarization states also causes polarization mode dispersion, which occurs because the two polarization modes have slightly different velocities. Polarization instability and mode dispersion degrade performance of optical fibers in some applications of single-mode fibers to communications and measurement systems.
In an optical communication system, the received signal level fluctuates when the receiver is sensitive to the polarization. This fluctuation occurs when an optical integrated circuit is used in the receiver and in heterodyne-type optical communications systems. Polarization instability manifests itself in optical interferometric systems in a manner analogous to signal fading in classical communications systems.
Slight elliptical deformation of the fiber may exist even when a fiber is designed to be axially symmetrical. Ellipticity separates the propagation constants of two orthogonally polarized HE.sub.11 modes, which otherwise are degenerate with each other, and causes polarization mode dispersion delay.
Single-polarization single-mode (SPSM) optical fibers were developed to prevent the adverse effects of polarization instability. Three basic types of the SPSM fiber are the elliptical-core fiber, the stress induced birefringent fiber, and the side-pit fiber.
Previous attempts to provide polarization stability have employed one of several methods of maximizing the differences between the propagation constants of the two polarization modes. Elliptical core fibers provide an asymmetrical propagation constant distribution to provide the required difference in propagation constant. Application of an asymmetrical stress distribution by bending a fiber will achieve the same result.
Elliptical core fibers are not practical because producing the desired birefringence in this manner increases the transmission loss to unacceptably high values and because of attendant difficulties in splicing such fibers together and in connecting them to other devices. Stress induced birefringence is subject to relaxation as the fiber optic material flows over extended time periods to relieve the stress. Stressing a fiber to induce birefringence also often results in a fracture of the fiber in the fabrication of fiber optic devices.